The present invention generally relates to the detecting of a heart response in an ECG signal of a patient having a pacemaker emitting a pace pulse.
In any Electrocardiogram (ECG) monitoring device, an important feature is the detection and characterization of each individual heart beat present in the ECG signal. This information is then used to generate both heart rate information and alarms in life threatening situations. Monitoring an ECG signal from a patient having a pacemaker is difficult as pace pulses generated by the pacemaker can occur at any time. When they occur between QRS complexes, they can be incorrectly detected by a QRS detector and result in an incorrect high heart rate measurement. When they occur during a QRS complex, they can cause incorrect feature measurement and result in an erroneous QRS classification. In particular, the detection of asystole is necessary to alert nurses of the cessation of heart response which is indicated by the absence of the QRS complex in the ECG signal. However, in the case of patients with pacemakers, the ECG signal, even after asystole, contains periodically occurring pace pulses, which may resemble heart response. The presence of pace pulses on an ECG signal makes it difficult to detect such asystole conditions.
FIG. 1A shows a typical pace pulse 10 consisting of two components, a main pulse 20 and pace pulse tail 30, sometimes also referred to as a re-polarization pulse. The main pulse 20, which is used to stimulate the heart, is characterized by its narrow width, sharp rise and fall, and large variation in amplitude. The actual shape of the pace pulse 10 mainly depends on the output coupling design of the pacemaker. The pace pulse tail 30 is used to deplete the capacitive coupling generated by the delivery of the pace pulse charge built up between the heart and the pacemaker. The shape and size of the pace pulse tail 30 are a function of the energy content of the pace pulse tail 20 and the amount of capacitive coupling. In addition to re-polarization, bandpass filtering in the monitoring equipment may create a further portion to the pace pulse tail 30.
FIG. 1B shows an example of a shape of a heart response 40. The heart response 40--also referred to as QRS complex--represents the response of the heart onto the stimulating pace pulse 10.
FIG. 1C shows an example of a shape of an actual ECG signal 50 which results from the pace pulse 10 superimposed with the heart response 40 of the patient. In other words, the synthetic pace pulse 10 stimulates the patient's heart response 40, which is superimposed with the pace pulse 10 to the ECG signal 50 to be measured by the ECG monitoring device. The ECG signal 50 comprises a positive pulse 60, mainly determined by the main pulse 20, and a negative pulse 70, mainly determined by the pace pulse tail 30 and the heart response 40.
Some pacemakers generate a pace pulse tail 30 with a substantially exponential decay (as indicated in FIG. 1A). In order to more accurately monitor ECG signals, it has been found helpful to eliminate pace pulse signals from the pacemaker. However, such elimination requires that the pace pulse 10 is first identified. It has been particularly found difficult to detect the heart response 40 on the pace pulse tail 30, especially if the energy delivered by the pacemaker is high. For a real-time system like a patient monitor, it is difficult to distinguish the signal form of the pace pulse tail 30.
The process of identifying pace pulses 10 may employ the technique disclosed in U.S. Pat. No. 4,664,116; wherein, pace pulses 10 are identified by the existence of high frequency "spikes" having narrow width and a sharp rise time, which exceed a minimum dynamic noise threshold.
Additional hardware and software can be employed to remove detected pace pulses 10. In particular, a technique is described in U.S. Pat. No. 4,832,041 in which values of the ECG signal 50 that are within a window containing the pace pulse 10 are replaced with substitute values that are an interpolation of selected values of the ECG signal 50. The substitute values form a line that is very close to what the ECG signal 50 would be if a pace pulse 10 had not occurred. However, this algorithm is not designed to eliminate the pace pulse tail 30. Unfortunately, the remaining energy of the pace pulse tail 30 may be erroneously detected as a QRS complex (heart response 40). This may cause the misdiagnosis of the patient's underlying ECG rhythm and result in a missed detection of an asystole condition.
U.S. Pat. No. 4,934,376 discloses a method and apparatus for detecting the occurrence of heartbeats in an ECG signal which may also include pacer artifacts, comprising analysis of the ECG signal for providing a heartbeat signal indicating detection of the occurrence of a heartbeat, analysis of the ECG signal for providing a pacer artifact signal indicating detection of the occurrence of a pacer artifact, analysis of the ECG signal in a manner independent from the first-mentioned analysis for determining if a portion of the ECG signal which follows detection of a pacer artifact has changes in its amplitude level which indicate the validity of the heartbeat indicating signal, and use of the result of the last-mentioned analysis to control the providing of the heartbeat indicating signal by the first-mentioned analysis.
U.S. Pat. No. 5,033,473, by the same applicant, discloses a method and apparatus for discriminating pace pulse tails 30 generated by signals discriminated from QRS complexes (heart response 40) by mathematically ascertaining that the signal following the pace pulse peak has an exponential decay. It is ascertained whether or not the waveform decays exponentially through application of a mathematical equation.